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V1 Media, the publisher of Informed Infrastructure Magazine, is an approved AIA continuing-education provider. AIA-approved courses are a valid form of Learning Units (LU) and Professional Development Hours (PDH) for the Architecture, Engineering and Construction (AEC) industries via self-learning courses in all states.

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Engineering a Permanent Road Foundation

Course Information

Unbound aggregate materials are the largest transportation infrastructure asset for most state and local Departments of Transportation (DOTs). This asset is used in road bases and subbases as well as for surfacing of unpaved roads. Unbound aggregate layers also are the most vulnerable transportation infrastructure, as these layers are subject to premature failure due to deterioration, contamination and adverse hydraulic conditions.

These “out of sight, out of mind,” underappreciated and often under-engineered layers are the subject of this PDH article. The loss of support of unbound aggregate results in full-depth reconstruction, a total loss of the aggregate assets, and road downtime with traffic congestion and construction-related traffic accidents.

Readers will be shown improved best-management design practices and how to preserve unbound aggregate assets to enhance the performance of these layers. Use of the correct aggregate and a separation/stabilization geotextile can construct a Permanent Road Foundation (PRF) so full-depth reclamation and total reconstruction may be avoided. The quality of aggregate enabled by the selection of the proper geotextile will maximize the cost effectiveness of these now-permanent road layers.

Demonstration is provided for how the structural and hydraulic benefits of nonwoven geotextiles make them the most cost-effective geosynthetic to be used in the establishment of a PRF. Comparison will be made between unbound aggregate PRF and other types of road support layers. The PRF will significantly lower the Life Cycle Cost (LCC) of the road. Overall Life Cycle Assessment (LCA) considerations also will be discussed.


David B. Andrews, P.E., and Mark Marienfeld, P.E.

Learning Objectives

• Why roads fail prematurely and how to assess the damage of unbound aggregate layers within their jurisdiction.

• The detrimental effects of road base layers becoming contaminated with subgrade soils.

• How to create a cost-effective Permanent Road Foundation (PRF) using a separation/stabilization geotextile.

• Why a PRF is the most durable and cost-effective road support layer compared to alternative strategies.

• How critical friction, durability and hydraulic properties uniquely position nonwoven geotextiles as the preferred geosynthetic to enable a PRF.

• How the preservation of unbound aggregate layers can dramatically lower the Life Cycle Cost (LCC) of transportation infrastructure.

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